1. Field of the Invention
The present invention relates to a catadioptric optical system, an optical pickup and an optical disk drive employing the optical focusing system, and a digital data storage medium on which information is recorded and from which information is read by the optical pickup and the optical disk drive.
2. Description of the Related Art
Various methods for increasing a recording capacity in an optical recording and/or reproducing apparatus have been studied. Those basic approaches are to reduce the wavelength of used light and to heighten a numerical aperture (NA) of a used objective lens, thereby reducing the size of a focused light spot. An optical focusing system for reducing the size of a focused light spot by heightening the numerical aperture is described with reference to FIG. 1.
The optical focusing system shown in FIG. 1 which reduces the size of a focused light spot using a near field, includes an aspherical lens 1 and a spherical lens 2 called a solid immersion lens. When the optical focusing system is used as an objective lens for an optical disk 4, a slider 3 carries the spherical lens 2 over the surface of the disk 4, and maintains the distance between the spherical lens 2 and the disk 4 to be less than 100 nm. The aspherical lens 1 refracts a laser light beam emitted from a light source (not shown), and the spherical lens 2 focuses the laser light beam refracted by the aspherical lens 1 on the inside of its surface faced to the disk 4. The surface of the spherical lens 2 on which the laser light beam is focused forms a near field, and thus information is recorded on or read from the disk 4 via the near field.
In the case that a material constituting the spherical lens 2 has a refractive index "n", in the inside of the spherical lens 2, the converging angle of a laser light beam becomes large and momentum of the laser light is reduced. Consequently, a wavelength of the laser light beam is reduced to .lambda./n. The numerical aperture (NA) is also increased to NA/.lambda.. Thus, a size of the light spot which is finally formed in the inside of the surface of the spherical lens 2 is proportional to NA/.lambda.. As a result, the size of the spot can be reduced using the refractive index "n" of the medium of the spherical lens 2.
However, the optical focusing system of FIG. 1 includes the aspherical lens 1 and the spherical lens 2 which are separately manufactured. Accordingly, it is hard to assemble or adjust the optical focusing system so that a desired optical characteristic is obtained. Since the optical focusing system requires incident laser light having a beam diameter more than or equal to 3 mm, the size of all optical components including a light reception unit are enlarged. As well, in the case where the inclination of incident beam that a laser beam deviates from a normally used angle with respect to an optical disk occurs due to sway of a moving optical pickup or a rotating optical disk, it is difficult to record or reproduce a signal normally.
Further, the shortest wavelength of light of a currently available laser diode light source is about 600 nm.
Also, the numerical aperture of the objective lens is approximately 0.6 at present. Thus, in the case where a numerical aperture of 0.6 or more is required, performance of an optical pickup is much sensitive to inclination of an incident beam. Accordingly, it is difficult to use the existing optical focusing system in order to commercialize an optical recording and/or reproducing apparatus.